Keiji Sakai

Characterization of vanadium oxide sol as a starting material for high rate intercalation cathodes

Abstract A novel solution process using vanadium oxide sol with acetone was developed and V2O5/carbon composite cathodes capable of high rate charge/discharge were fabricated. It was found that the sol condition affects the electrochemical performance of the cathodes. This report presents the fabrication process and the electrochemical performance of the cathodes, and the morphological property of the colloidal particle such colloidal particle size, density and shape at various concentrations. The correlation between the electrochemical performance of the cathodes and the colloidal character of the sol is discussed.

Acoustic phase conjugation by nonlinear piezoelectricity. II. Visualization and application to imaging systems

Phase conjugate waves of ultrasound were generated in PZT ceramics through nonlinear piezoelectric interaction between an incident ultrasonic field at ω and an electric field at 2ω. The amplitude reflectivity of the phase conjugator was 23% at 10 MHz. The behavior of the incident waves and the phase conjugate waves were visualized by stroboscopic schlieren technique. Time-reversal property and the automatic correction of wavefront distortion in the phase conjugate process were confirmed. A scanning ultrasonic imaging system with a PZT phase conjugator has been built. This system was used to visualize test samples composed of solid plates and phase disturbers. Images by phase conjugate reflection yielded clear figures of the solid plates in spite of the existence of phase disturbers made of agarose gel with rough surfaces, whereas conventional images showed serious distortion.

Electric field tweezers for characterization of liquid surface

The authors have developed a novel technique to measure the surface properties of soft condensed matters in a noncontact manner. In this method, the Maxwell stress applied by a needle electrode deforms the surface. The magnitude of the displacement is determined by the surface tension as the restoring force, while its dynamic response is determined by the sample viscosity. The surface tension was measured for the aqueous solutions of the surfactant sodium dodecyl sulfate at various concentrations and the result shows good agreement with the conventional method. Further, the viscosity measurement was also carried out in the range of 0.1–103Pas.

Measurement of high viscosity with laser induced surface deformation technique

A technique for viscosity measurement was developed based on the principle of laser-induced surface deformation. Light incident into liquids increases its momentum due to the difference in refractive index and gives the surface an upward force as a reaction. The plane surface thus swells up and deforms, and the shape is determined so that the force is balanced with the surface tension and the gravity. On sudden laser irradiation, the deformation inevitably accompanies a viscous flow and exhibits a relaxational behavior with a delay time, which gives the viscosity. Theoretical prediction of the step-response function was given that takes surface tension waves excited by the laser into consideration. Nd–yttritium–aluminum–garnet laser with 0.6W output was focused to ∼200μm beam waist and used for the pumping. The deformation process was observed sensitively with another probe laser illuminating the activated area. This system was tested with the standard liquids for viscosity ranging from 1 to 106cSt. The r...

Electromagnetically Spinning Sphere Viscometer

We have developed a simple system for viscosity measurement in the range of 10-3 to 101 Pas. An aluminum sphere revolves in a sample cell under a rotating external magnetic field, and the rotational speed of the sphere gives the sample viscosity. Low viscosities can be measured by using a micro-probe sphere, since the viscous torque applied to the sphere overcomes the frictional force at a sufficiently small sphere radius. The system is free from contamination and can be easily applied to medical and biological studies.

Thermal Phonon Resonance in Solid Glass

Thermal phonon resonance was observed in a stiff solid material of fused silica, from which a lower intensity of scattered light is expected than from liquids. The improved detection sensitivity of the optical beating Brillouin spectroscopy technique enabled us to observe the spontaneous elastic resonance of the solid sample attributable to thermal density fluctuation with a frequency resolution of 1 kHz. The accuracy in determining phonon velocity was then increased up to 10-5. The high frequency resolution also revealed the fine structure of the resonance peak train brought about by the lateral mode vibration, and the eigen frequencies of the compound mode are in good agreement with those theoretically estimated. Volume and shear elasticities were uniquely determined from the resonance spectra of longitudinal and shear phonons. Phonon absorption at about 100 MHz was also determined from the resonance peak width of the Brillouin component.

Observation of collision and oscillation of microdroplets with extremely large shear deformation

We measured the viscosity and surface tension of various liquids under large (∼106 s−1) shear deformation. Oscillation of a 10-μm size microdroplet is brought about by the head-on collision of two droplets. Since the Reynolds number is as small as 100, the motion of the liquid is stable and the dynamic image is obtained with high reproducibility by the stroboscopic method. By observing and evaluating the mechanical oscillation of the microdroplet, of which frequency ranges typically in 100 – 300 kHz, we found that the viscosity of ethylene glycol and diethylene glycol is smaller than the known literature value, which is considered to be the viscosity at zero-frequency. This phenomena can be attributed to the slow viscous relaxation of associated liquids due to the re-combination dynamics of the network of H-bonds.

Acoustic phase conjugation by nonlinear piezoelectricity. I. Principle and basic experiments

The principle and the basic experimental results of the acoustic phase conjugation by nonlinear piezoelectricity are described. Acoustic phase conjugate waves at frequency ω are generated as a result of the interaction between incident acoustic waves at ω and pump electric fields at 2ω. An original explanation based on the modulation of sound velocity by the electric field together with the concept of time grating is given. Coupling equations for the PZT ceramics as a nonlinear material are derived. Experimental results of the sound velocity modulation and the phase conjugate generation in nonlinear piezoelectric PZT ceramics are shown. Amplitude reflectivity of the phase conjugation was 23% for the acoustic incidence at 10 MHz.

Friction torque reduction by ultrasonic vibration and its application to electromagnetically spinning viscometer

The technology of using ultrasonic vibration to reduce friction was applied in our experiment to improve the accuracy of viscosity measurements. The electromagnetically spinning (EMS) viscometer we developed has a remarkable feature: the sample viscosity can be measured in an entirely noncontact manner. Although the apparatus enables low-viscosity measurements of less than 10 mPa·s, which has been difficult to achieve using conventional rotation viscometers, the accuracy has been limited to 10% because of the harmful effect of mechanical friction. We made a new EMS system equipped with piezoactuators to excite ultrasonic vibration to the sample cell, thereby successfully reducing the mechanical friction to one-third that in the previous system. A theory to describe the mechanical friction between the rotating probe and oscillating substrate was also proposed and examined in comparison with the experimental results.

Low-Viscosity Measurement by Capillary Electromagnetically Spinning Technique

We study sphere rotation against viscous torque confined in a small space. Our new invention, the electromagnetically spinning sphere (EMS) viscometer measures liquid viscosity through the observation of sphere rotation driven by electromagnetic interaction in a noncontact manner. The lower limit of the measurable viscosity is determined from the ratio between the viscous torque and the mechanical friction, and the apparent increase in the contribution of the viscous term leads to the improvement of the accuracy of low-viscosity measurement. We propose a theoretical expression of the torque applied to a sphere rotating in a cylinder and obtained the power law with respect to the gap in between. The results of the numerical simulation and experiment provide evidence of the validity of the theory.